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Many of Microsoft's online properties are unreachable Wednesday morning, the software giant confirms. The disruption is due to a problem in the system that maps Web addresses to IP (Internet Protocol) addresses, Microsoft says.

Because of the blackout, some 60 million Hotmail users worldwide can't access their e-mail, Microsoft customers can't download software updates or get online support, and news Web site MSNBC.com has no audience.

Other services that can't be reached include Web portal MSN.com, Windowsupdate.com, which contains updates for the Windows operating system, Passport.com, Microsoft's online identification service, and BCentral.com, a portal for small and medium-size businesses.

"The Internet's Domain Name System (DNS) does not return the correct response when it is queried for a Microsoft Web site," says Ruud de Jonge, support manager at Microsoft Benelux.

The first reports of the problem started coming in "very early" Wednesday morning, says De Jonge. It is unclear when the sites will be available again. "It will take some time; this can't be restored by hitting one switch," he says.

Microsoft has yet to pin down the cause of the DNS error. "It can be a system or human error, but somebody could also have done this intentionally," De Jonge says. "We don't manage the DNS ourselves, it is a system controlled by the Internet Corporation for Assigned Names and Numbers (ICANN) with worldwide replicas."

A team at Microsoft's headquarters in Redmond, Washington, is working on the problem, which has been given top priority, says De Jonge.

Hackers could be responsible for the outage that has continued for hours now, says Simon Hania, spokesperson for Dutch Internet service provider XS4ALL Internet BV. "The name server that is authoritative for Microsoft's Web sites might have crippled under a denial-of-service attack," he says.

Hania says, however, it is more likely that a network error or system failure caused the problem.

The DNS consists of many machines around the world set up in a hierarchy. "It looks like the machine hit is in the top of the DNS tree," Hania says. "Once it is fixed it can take a couple of hours for all DNS systems around the world to pick up the correct DNS information."

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